The need to reduce weight and eliminate corrosion problems has led the aeronautical industry to make commercial aircraft fuselages of composite material.
Of particular importance amongst the structural elements which make up the fuselage are the circumferential frames which are connected to the outer skin by means of another structural element called a shear tie, stabilizing the outer skin and giving it the strength to resist flight and pressurization loads.
The current technology provides for various processes for manufacturing the composite frames and shear ties separately. They are then riveted together and then to the outer skin to create the final assembly.
A particularly advanced technique, however, is to produce the integral frame and shear-tie assembly as a single component so as to be able to reduce its weight (the absence of riveting enables thicknesses to be reduced in the joining regions and the weight of the connecting members is avoided) as well as costs and flow times.
Up to now, the production of integral frames and shear ties has been achieved by a resin-transfer moulding process (RTM) which enables dry carbon-fibre reinforcement preforms to be infused with resin; the preforms are produced from dry, unidirectional fabric layers which are preassembled and positioned in the infusion and curing mould.
This process has the following disadvantages:                to enable the resin to flow in the RTM process, it is necessary to use very fluid resins which, however, cannot contain within them the toughening elements that are necessary to improve their properties, such as compression after impact or low inflammability; an alternative is to use resins the toughening of which is woven into the preform and then dissolves in the resin at the time of infusion; however, this process makes the production of the preform more complex and expensive;        the materials used for the reinforcement have straight fibres which are unsuitable for producing curved structural elements; in fact their forced adaptation to curved shapes causes uncontrolled wrinkles and/or distortions of the orientations of the fibres, resulting in unreliability of the mechanical properties;        structural optimization often requires the circumferential elements to have a certain percentage of reinforcing fibres that are arranged in a curved manner; this requirement cannot be satisfied with the reinforcements that are used in the current state of the art;        the tools for RTM are complex to assemble, to hold in position with the necessary accuracy during the injection of resin, and to disassemble after polymerization, and are difficult to clean and maintain.        